摘要
聚合物降解在原子层次上的理论研究是相关于高分子材料反应调控的重要课题．运用引入色散修正的密度泛函紧束缚理论（DFTB-D）方法，对两端不饱和与一端不饱和（c-端不饱和，CH2-端不饱和）共3种典型顺次连接的聚-a-甲基苯乙烯（PAMS）片段的降解过程进行了原子层次动力学模拟研究．结果显示，PAMS在500～600K的温度环境下，降解都对应于解聚过程，并且单体单元逐一脱落主要发生在链的未饱和端部．进一步的3种片段的电子结构分析均显示，占据在最高占据分子轨道（HOMO）和最低非占据分子轨道（LUMO）的电子主要局域在未饱和一端，这与势能面预测的结论是相符的．此外，动力学模拟的结果也指出，适当的增加温度，能够让解聚反应加快进行．进一步基于DFFB—D方法的C-C键断裂过程的弛豫扫描也清晰地显示，从能量角度，解聚发生在不饱和端也更为容易．此外，自旋布居分析体现了这是与PAMS的电子自旋极化结构紧密相关的．我们希望，当前的理论研究能够对理解聚合物降解机理起到基本的参考作用．
Polymers are macromolecus composed of repeated monomer subunits, which construct the fabulous world of DNA, protein, cellulose, rubber, plastic, etc. and possess crucial positions in fields like life science, chemistry industry and material preparation with special demands. However, the performance of polymer would decay because of the influence of environment effects and even be depolymerized. While under particular scenarios, such a decomposition process might be valuable. Poly-alpha-methylstyrene （PAMS） is the substrate materials for preparing glow discharge polymer （GDP） shell which coats the fuel target for inertial confinement fusion. The key step for its involvement is to depolymerize into single molecules under high temperature so that it can escape through the space among GDP molecules which has deposited on it, and then leaves only the GDP shell. Besides of this special application, the atomic level understanding of depolymerization processes is also at the key position to modulate other reactions involving polymer materials. Here, through molecular dynamic simulation based on density functional tight-binding methods containing dispersion correction （DFTB-D）, we demonstrate the typical dissociation of sequential poly-alpha- methylstyrene （PAMS） tetramer fragments with unsaturation on both ends, on the C- end and on the CHz- end, respectively. DFTB method we used here is an approximation based on the second order expansion of the density functional theory （DFT） total energy with respect to charge density variation relative to a chosen reference density. It promotes the efficiency of DFT method around two to three orders meanwhile remains acceptable precision for electronic structure calculation and large-scale quantum dynamic simulations. The reliability of the method has been admitted in massive researches, especially those on carbon-based molecules and organic systems. Our results show, with the temperature of 500 and 600 K, the dissociation of PAMS fragments is implemented by depolymerization processes, where monomers separate from one of the unsaturated ends one by one, and rising temperature could reasonably accelerate the reaction. Further simulations of a longer hexamer PAMS fragment under 600 K indicates the length effect won＇t cause qualitative influence on the depolymerization process. The electronic structures of these three fragments indicate both the highest occupied molecular orbital （HOMO） and the lowest unoccupied molecular orbital （LUMO） are localized at the bond-breaking unsaturated ends, which supports the prediction of dynamic simulations for reaction sits. Furthermore, the DFTB-D electronic structure calculations following the C-C bond breaking steps at different possible sites along the backbone chain also show the preference of the reaction to unsaturated end sites, from the view of potential energy surface. Spin distribution analysis also reflects the energy curves respect to the different depolymerization processes are closely related to the spin-polarized electronic structures of corresponding products. These results give a representative atomic-level prospect about the decomposition process of sequential PAMS under high temperature with detailed interpretation on the reaction sit preference and order, which may be informative for preparation of relative device. Besides, we hope this theoretical study could offer a fundamental reference for understanding the mechanism of decomposition for polymer materials beyond PAMS and spread across the wonderful microscopic world.
出处
《科学通报》
EI
CAS
CSCD
北大核心
2016年第21期2371-2379,共9页
Chinese Science Bulletin
基金
国家自然科学基金(11374004)
吉林省科技发展计划(20150519021JH)
霍英东青年教师基金(142001)资助
